6.4.2 Tube support plates
The tubes are supported by a series of tube support plates in the form of mild steel drilled with the same pattern of holes as the tubeplate. The hole size is a close fit over the outside diameter of the tubes. The clearance between tube and tubeplate varies according to the manufacturer's practice, but general guidance is given in TEMA 1978 .
The tube support plates are distributed along the length of the tube bundle to meet three requirements:
- To guide the steam or water across the tubes to provide efficient heat transfer.
- To support the tubes so that they are not damaged by the fluid flow expected to occur within the heater shell.
- To form the ends of the drain cooling and desuperheating section.
Figure 3.50 shows the layout of the tube support plates within a HP heater with drain cooling, condensing and desuperheating sections. The sketch illustrates how the tube support plates act as baffles to distribute and guide the steam as it flows through the desuperheating section and into the condensing section. The drains water is guided through the drain cooling section by another series of baffles. Initial maximum permissible values of baffle pitch are usually found from a design code, such as ТЕМА .
In any tubular heat exchanger with fluid flow across the tubes, there is the possibility of tube damage due to flow-induced vibration. When fluid flows perpendicular to a cylindrical object, vortices are periodically shed downstream, the shedding frequency being proportional to the fluid velocity. The cylindrical object will vibrate and if the shedding frequency is equal to the natural frequency of the object, a resonant condition will occur. There is also the possibility that because tubes are long slender objects, they may under certain conditions be excited by random eddies from vortices within the tube bundle and a resonant vibration could occur. These phenomena are the basis for a method given by Thorngren , which calculates two dimensionless damage numbers NBD and NCD.
NBD is a factor which determines the possibilities of damage due to fatigue of the tube material at the baffle hole.
NCD is a factor which is used to predict if collision damage will occur between adjacent tubes.
If the damage numbers are both less than unity, then the distance between baffle plate supports is adequate. If the damage numbers are greater than unity, then the distance between the baffles needs to be decreased. However, reducing the distance decreases the steam flow area and therefore increases the velocity, which in turn influences thermal design. For further information, reference can be made to Section 12 'Recommended Good Practice' of the ТЕМА 1978  which gives further references and advice.
The baffles are spaced either by means of tie rods, which take the form of long screwed bars projecting from the tubeplate with the baffles set at fixed distances apart by tubular spacers at appropriate intervals, or by welding them to a structure within the heater shell which is anchored to the heater tubeplate. The baffles are close together in the desuperheating and drain cooling sections, but wider apart in the condensing section to achieve the best velocity profile and to maximise the heat transfer process. The ideal arrangement of baffles is only found by an iterative process involving both thermal and mechanical design. The relevant aspects of the thermal design are found in Section 6 of this chapter.